How Do We Use Terrestrial Laser Scanning in Hydraulic Models

“How do we use terrestrial laser scanning in hydraulic models?”

“The Hepple Lecture Theatre”

School of Geographical Sciences, University Rd., BristolBS8 1SS

Wednesday 21 October 2009

Attendees:

Environment Agency
Geomatics Group, Co-Hosts / Academic Community
Al Duncan (Speaker) / Albert Cheng, Exeter
Patrick Hogarth / David Mason, Reading
Andrew Richman / Pauline Miller, Newcastle (Speaker)
Paul Smith / Adrian Saul, Sheffield (Speaker)
Adam Tobin / Nigel Wright, UNESCO IHE/Leeds
Environment Agency / University of Bristol, Co-Hosts
Chris Clarke, FRM&DM– Midlands Central / Paul Bates (Chairman)
Richard Groom, FRM&DM - Thames West / Guy Schumann
Niall Hall, FRM&DM- Midlands West / Jeff Neal
Helen James, FCRM Mapping & Modelling / Tim Fewtrell
Sam Jamieson, FRM&DM- ThamesSE / Mark Trigg
Claire Jarman, FRM&DM- SW Wessex / Liz Stephens
Julian Pillans, FRM&DM -Thames SE / Amanda Hall
Rupert Spark, FRM&DM- SW Wessex / Miguel Rico
Dawei Han
Consultants / Local Authorities
Mike Panzeri, HR Wallingford / Tony Donaldson, Derby Council
Neil Hunter, JBA Consulting / Syd Simpson, City of Bradford
Matt Horrit, Halcrow / Dave Stewart, Torbay Council (Speaker)
Ian Anderson, Halcrow / Nick Tolley, Derby Council
Sun Yan Evans, Mott MacDonald / Mike Wood, Torbay Council
Richard Allitt, RA Associates
Bruce Kerridge, Capita Symonds / UKWIR
MichaelLake, Capita Symonds
Nathan Muggeridge, Mouchel / Barry Luck, Southern Water

Apologies:

Environment Agency / Academic Community
Mel Andrews, FCRM Inland Overview / Slobodan Djordjevic, Exeter
Mike Coward, FRM&DM – NW North / Garry Pender, Heriot Watt
Jim Walker, Flood Risk Science / Martin Smith, Nottingham
Consultants
David Hetherington, Arup
David Martin, Ambiental

Introduction to Mobile Terrestrial LIDAR

Al Duncan – Geomatics Group

• The Environment Agency Geomatics Group is synonymous with delivering the highest quality, most accurate height data in the UK.Once considered prohibitively expensive, LIDAR height data is now widely available at an affordable price and has become a vital component for effective modelling, planning and asset management for many organisations.

• Geomatics Group have invested in a Mobile Terrestrial LIDAR (an Optech LYNXTM). This vehicle mounted laser scannercaptures very high resolution data and provides exceptional levels of accuracy. This system allows for the collection of thousands of heighted co-ordinates per square metre within view of the sensor, with a 360o field-of-view, mobile terrestrial LIDAR is ideally suited for surveying roads, bridges, buildings, assets such as street furniture, and power lines.

• Description of sample data (previously supplied). This included the DSM with all surface objects, DSM including objects of wall and vehicle height, DSM of kerb height objects and the DTM of ground classified only points.

• Manual editing is then required to produce an enhanced DTM with kerb lines and examples of this were shown at various resolution (10, 25, 50 and 100 cm) with a view ot lowering file sizes versus requirements for modelling.

• The intensity image may also be useful in identifying drains and manholes. The data can be combined with other mapping sources such as OS MasterMap. 3d vectors can be derived from digitising features in the intensity image and the DEM data.

• Examples were presented of the Point cloud data product,rendered in 3D with colour to represent relative height and the brightness of shading intensity.

Questions & Answers

Do you have problems with backscatter?

The main technical issue comes from shadows / data gaps behind vehicles and walls. This then requires interpolation in order to generate a continuous terrain model.

Can you take the vehicle off-road?

Yes – the instrument can also be mounted on the most appropriate vehicle for specific surveys.

Is the accuracy different from airborne LIDAR?

Yes, absolute accuracy of mobile terrestrial Lynx data is 3 – 5 cm x, y and 3 – 5 cm z (relative accuracy 1 – 3 cm) RMSE.

Absolute accuracy of airborne LIDAR 40cm x, y and5 - 15 cm z RMSE.

Does the type of vehicle affect quality?

The instrument can be mounted on any vehicle. However, increased height will not reduce shadowing significantly. Our operational height of 2.5m was chosen to take account of bridges and overhanging trees.

Have you tried to remove vehicles from the streets prior to survey?

Not as yet but if this were possible, it would reduce the level of shadowing / reflecting and resultant interpolation in the terrain model.

Do you have any idea of error sources?

We would only survey in dry conditions so there should be no problems with signal response.

The vehicle’s position is monitored 200 times per second so pot holes etc are accounted for. (Post workshop comment: error sources include the GPS/IMU/Range/ temperature and surface reflectance.)

Why use vehicle LIDAR – can you quantify detail / accuracy / costs?

Deployment of Mobile Terrestrial LIDAR is cheaper than that of airborne LIDAR. Precise costs will be dependant on the products / outputs required by the customer. (Cost estimate to be provided with Glasgow test site data.)

Urban Flood Modelling

Adrian Saul - University of Sheffield

• An integrated approach to urban drainage is vital to urban flood modelling within integrated catchments.

• The research challenge is to develop a generalised tool to deal with the interactions of any above ground flows and their interaction with the below ground drainage system

• The foundation for successful modelling is an extremely accurate urban surface DEM/DTM - sophisticated data is needed to understand complex flows.

• Street models are vital - without the precise positions of gulleys and manholes, kerbs, walls, fences, permeable structures and other infrastructure affecting the flow of water, the accuracy of a model is compromised.

• The Flood Risk Management Research Consortium (FRMRC) has developed new urban interactive model which has been trialled in three UKWIR funded case studies – includingTorbay. These have highlighted the need for a joint 1D/1D and 1D/2D approaches. The detail of the catchment surface is the critical factor and enhanced DEM’s are the way forward.

• Mobile terrestrial LIDAR data has greatest potential in the intra-urban zone where fluvial flows are just a minor consideration.

• Mobile terrestrial LIDAR data needs to be supported by topographical / historical data to in order to validate the models produced.

Questions & Answers

Why not throw all the data into a 1D/2D model?

Whilst it is easy to collect the data required for 1D/2D modelling, it very time-consuming to process. For this reason the combined approach, which also employs faster 1D/1D models, has been adopted.

What resolution of data do you/should we use?

As high as feasibly possible. Don’t confuse the resolution of models with that of the data! High resolution data can be used to identify features for input into models which use a coarser grid resolution.

Models will only be as good as the information used to create them.

You highlighted the importance of DTM’s for overland processes – how much do we know about our underground drainage systems which also play an important role in modelling?

Our understanding of drainage systems has improved over the last ten

years, particularly in those areas at risk. Models are often held by

water companies, although they are closely protected for legislative reasons.

Further general discussion comments:

DTM’s need to be considered as an asset which is continually improved over time.

Ground data (site walk by engineer) is and will always remain vital to determine the success of a model and for verification.

Data and Models:

University of Bristol have run models for Tewkesbury at 1m resolution in five to six days.

Environment Agency models of Tewkesbury at 10m resolution took ten days to run.

Consultants tend to restrict the resolution of data to 4m when working with the Environment Agency.

CCTV can be used to verify the overland flow model

Model application in Torquay with 1D & 2D surface flow modelling

Dave Stewart – Torbay Council

• Presentation of work using the FRMRC urban interactive model (UKWIR funded) showing the application of the data and modelling provided by the team. – see slides.

• CCTV was presented showing how it could possibly be used toverify the overland flow model.

Terrestrial Laser Scanning: Applications in Civil Engineering

Pauline Miller – NewcastleUniversity

• Highlighted that we are lookingat a technique, that can be applied using data from a variety of terrestrial laser scanners.

Plenary Session

Paul Bates – University of Bristol

Four short presentations from members of the audience on experience with surface water modelling:

Nathan Muggeridgewith Mouchel software. Chris Clark (Environment Agency) highlighted the importance of site survey walking for verification with an example of introducing garden fences as breaklines to improve the model output against the event outlines. Matt Horrit with Halcrow ISIS-Fast software using the 10cm sample Lynx data. Tim FewtrellBristol University LISFLOOD-FP software also using 10cm sample Lynx data.

General comments:

10cm sample data independently used by different people for the modelling examples, probably a good resolution for the hydraulic modelling.

Elevation data is now widely available and the right resolutions. There is a need for other model input data to improve, e.g. rainfall predictions, river flow monitoring calibrationand validation data, drainage knowledge and to a lesser extent surface roughness.

Mobile terrestrial LIDARshould prove useful in providing the break lines (curbs etc) that are vital in hydraulic street models and for adding such features to airborne elevation data.

The issues of merging mobile terrestrial and airborne LIDAR data were discussed.

The costs of mobile terrestrial data vs. airborne data andairborne data at 25cm vs. 50cm need to be explored with the test site data capture.

Post workshop notes:

• Airborne data resolution for flood modelling:

50cm for urban areas, 1m for floodplain, 2m for rural areas outside of the flood plain.

• Comments noted during the lunch time session on the RGB colour coding of the airborne LIDAR with digital photography was that this provided an excellent visualisation of thedata and area of interest.

Geomatics GroupSchool of Geographical Sciences